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2009, 113, 3899–3902 Published on Web 02/16/2009
Photostimulated Reversible Attachment of Gold Nanoparticles on Multiwalled Carbon Nanotubes Zhimin Wang,*,† Zhan-xian Li,‡ and Zhaohui Liu† School of Science, Henan Agricultural UniVersity, Zhengzhou 450002, People’s Republic of China, and Department of Chemistry, Zhengzhou UniVersity, Zhengzhou 450001, People’s Republic of China ReceiVed: January 4, 2009; ReVised Manuscript ReceiVed: February 8, 2009
In this study, a hybrid system of gold nanoparticles and multiwalled carbon nanotubes (MWNTs) with a switching function was successfully realized through photocontrolled host-guest interaction based on R-cyclodextrin (R-CD) and azobenzene derivatives. First, the azobenzene derivative was covalently bonded onto the surface of MWNTs by a simple and effective reaction, leading to highly water-soluble MWNTs. Then, narrowly distributed gold nanoparticles were prepared using thiolated R-CD as a capping agent. Because of the photochemically controlled inclusion-exclusion interaction between the azobenzene moiety and R-CD, the reversible attachment of gold nanoparticles onto the MWNT surface was realized. TEM observation traced and testified to this reversible process, and at the same time Raman spectra provided convincing proof of the photoisomerization process. Carbon nanotubes (CNTs) have been expected in many possible applications because of their precious electrical, thermal, and optical properties1 and extremely high mechanical strength and stability.2 To optimize their use in many applications, it is advantageous or required to modify carbon nanotubes by physical or chemical methods.3 Among them, the hybridization of carbon nanotubes with metal or semiconductor nanoparticles is an important direction in functionalization, which could endow the hybrids with some exciting features such as enhanced catalytic activity4 and electronic5 and other useful properties.6 These properties mean that the hybrids hold promise in electronic, optical, and magnetic applications.7 During the past few years, much effort has been expended in this field to explore effective strategies for the attachment of nanoparticles on carbon nanotubes, and great progress has been achieved.8 So far, all of the attention has been focused on the attachment of nanoparticles to carbon nanotubes. However, there are no reports concerning the detachment of nanoparticles from hybrids. It would be significant to develop a smart carbon nanotube/ nanoparticle hybrid system with a switching function that could control the nanoparticle attachment onto and detachment from carbon nanotubes by an external stimulus. Such hybrid should be important in modulating the electronic properties of carbon nanotubes reversibly, in heterogeneous catalysis, in controlled loading and unloading, and so on. It is well known that CDs and their derivatives can form inclusion complexes with a number of complementary azobenzene compounds via host-guest recognition.9 Under common conditions, the azobenzene derivative adopts a trans conformation, which is thermodynamically more stable than cis-azobenzene and can form an inclusion complex with R-CD through * To whom correspondence should be addressed. E-mail: gary1451@ iccas.ac.cn. † Henan Agricultural University. ‡ Zhengzhou University.
10.1021/jp900055z CCC: $40.75
Figure 1. Schematic presentation of gold nanoparticles’ reversible attachment onto and detachment from the surface of MWNTs through the photochemically controlled host-guest molecular recognition interaction.
host-guest interaction. Irradiation with ultraviolet light of about 360 nm leads to isomerization from the trans to the cis isomer, inducing a drastic change in its molecular occupied volume that can modulate the inclusion and exclusion as a result of the stability of host-guest complexes.10 Reversible trans-cis transformation can occur upon alternatively irradiation with visible and UV light. This robust control of the transformation of conformation via light irradiation is advantageous for application to switchable building blocks, such as molecular shuttles, motors, and information storage in various matrices.11 Inspired by this reversible inclusion interaction, we wondered if inorganic nanoparticles could be supported reversibly on CNTs based upon the CD/azobenzene interaction. As a proof of principle, in this study we demonstrated for the first time that gold nanoparticles capped with R-CDs could be attached onto and detached from the surface of modified carbon nanotubes reversibly upon alternate UV irradiation and darkness as illustrated in Figure 1. 2009 American Chemical Society
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Figure 2. Azobenzene-derivative-functionalized MWNTs by the reaction between pristine MWNTs and 4-amino-azobenzene-4′-sodium sulfonic acid.
Figure 3. Dispersion state of the functionalized MWNTs in water. (a) TEM images of the modified MWNTs in water, with the inset showing a macroscopically homogeneous dispersion of MWNTs. (b) Magnified views from the marked location, with the position marked by arrows showing a clear, uniform layer from the bonded azobenzene derivatives.
For a clearer display of the reversible loading process, thicker pristine MWNTs with diameters of 30-50 nm and lengths of several micrometers were used in this study. At first, the pristine MWNTs were functionalized with an azobenzene derivative through a simple, effective method developed by Tour’s group.12 The detailed reaction process and the structure of azobenzenederivative-functionalized MWNTs are shown in Figure 2. The resultant functionalized MWNTs were very soluble in water, and the solubility could reach 1 mg/mL. As shown in the Figure 3a inset, the functionalized MWNTs could be dissolved in water to form a macroscopically homogeneous dispersion, which could be diluted to a transparent solution without precipitation for at least six months. To examine the dispersion status of the modified MWNTs in solutions, one drop of the water solution with redissolved functionalized MWNTs (0.5 mg/mL) was dropped onto a copper grid coated with a carbon film for TEM analysis. As shown in Figure 3a, the exfoliated individual MWNTs could be clearly observed from the TEM image. From the magnified view in Figure 3b, it is demonstrated that a clear layer covering the surfaces of MWNTs, which is close to the size of the azobenzene derivative, indicates successful modification. Thermogravimetric analysis (TGA) is a powerful tool in characterizing the functionalization of carbon nanotubes. As shown in Figure 4a, the modified MWNTs have significant weight loss compared to the pristine MWNTs below 700 °C. In comparing the thermograms of the modified MWNTs with that of the free azobenzene derivative, a similar thermal decomposition process is observed in the temperature range, indicating the successful functionalization. According to the TGA traces, the grafting content of the azobenzene derivative on the surfaces of MWNTs is about 35 wt %. To give further evidence of covalent modification, X-ray photoelectron spectroscopy (XPS) and elemental analysis were conducted to determine the element constituents and relative
proportions of the modified MWNTs. From the results of XPS (Figure 4b), N, S, O, and Na elements could be identified clearly, and the relative proportion was close to the stoichiometry of the azobenzene derivative bonded to the MWNTs. Elemental analysis gave consistent results: for functionalized MWNTs, the contents of N and H were 3.5 and 1.7 wt %, respectively; however, for the pristine MWNTs, the contents of N and H were almost undetectable, demonstrating the successful introduction of azobenzene groups. The R-CD-capped gold nanoparticles used in this work were prepared by the reduction of HAuCl4 in DMF solution containing thiolated R-CDs, SH-R-CD, according to the reported method.13 To obtain small particles, a smaller [Au3+]/[SH-RCD] ratio of 2:1 was used in our experiment. As shown in Figure 5, small R-CD-capped gold nanoparticles (ca. 3 nm) with a narrow distribution were prepared. The average numbers of R-CD molecules on the surface of a particle can be estimated according to the result of thermogravimetric analysis (TGA) and the assumption that a 3 nm gold cluster contain ca. 900 atoms.14 The content of R-CD in the R-CD-capped gold nanoparticle determined by TGA was ca. 30 wt %. Thus, the average number of R-CD molecules bonded to the surface of a particle was estimated to be about five to six. The preparation process of R-CD-capped nanoparticles was traced by FTIR spectra (Supporting Information, Figure S1). The spectral features are identical to those recorded with the free SH-R-CD apart from the disappearance of the S-H stretch at 2560 cm-1 in the spectra of the modified nanoparticles, as anticipated because of the formation of covalent bonds between the thiol functional group and the gold particle surface. This is consistent with the results reported in the literature.15 The results obtained above indicate clearly that the azobenzene-derivative-modified MWNTs and R-CD-capped gold nanoparticles were successfully prepared. To validate whether the gold nanoparticles could reversibly attach on and detach from the surfaces of MWNTs, the two components were mixed together in water in an [azo]/[Au particle] ratio of 1:2. As shown in Figure 6a, gold nanoparticles could be successfully attached onto the sidewalls of MWNTs after stirring the mixture for 12 h. Under this condition, the -NdN- bond of the azobenzene derivative takes a trans conformation. It was apparent that the gold nanoparticles were mainly located on the tube surface. Intriguingly, the hybrid composite was also soluble and could retain homogeneous, transparent solution for at least 3 months. To examine whether the immobilized gold nanoparticles could be detached from the surface of MWNTs, the mixture was irradiated with UV light (365 nm, 3.5 mW/cm2) for 3 h. From Figure 6b, we could see that the gold nanoparticles were completely detached from the surfaces of MWNTs and dispersed around the MWNT, indicating the exclusion of gold nanoparticles due to the photoisomorization of the azobenzene deriva-
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Figure 4. (a) TGA curves of pristine MWNTs, functionalized MWNTs, and the free azobenzene derivative and (b) XPS analysis of the functionalized MWNTs.
Figure 5. R-CD-capped gold nanoparticles dispersed in water. (a) TEM image and (b) size distribution.
Figure 6. TEM images of the photocontrolled reversible attachment of gold nanoparticles on azobenzene-derivative-modified MWNTs. (a) Mixing stoichiometric R-CD-capped gold nanoparticles with functionalized MWNTs in water and stirring for 12 h, (b) UV irradiated for 3 h, and (c) standing for 24 h in the dark.
tive. To demonstrate the detachment process clearly, the UVirradiated solution was centrifuged at 3000 r/min for 30 min. As shown in Figure S2, almost no gold nanoparticles existed in the solution, indicating the complete detachment of gold nanoparticles from MWNTs after UV irradiation. If we let the UV-irradiated solution stand in the dark for 24 h, then the detached gold nanoparticles could reattach onto the surfaces of MWNTs (shown in Figure 6c), demonstrating the reversible character of this process. Host-guest interaction is a kind of simple, effective driving force in the field of supramolecular self-assembly, which acts as an important role in nanoscience and nanotechnology for constructing functional components. In this study, noncovalent host-guest interaction was subtly utilized to create an MWNT/ gold nanoparticle nanosystem, which exhibited a switchable function. To prove that the reversible attachment and detachment process was driven by the photoisomerization process of the azobenzene derivative, Raman spectra were traced directly in the solution phase. Raman spectral measurements of the mixture solution of the azobenzene-derivative-modified MWNTs and the
R-CD-capped gold nanoparticles before and after UV irradiation were performed with a JY-HR800 (France) Raman apparatus using a 532 nm laser beam with a laser power of 5 mW and a detector data acquisition time of 100 s. Before UV irradiation, the azobenzene derivative adopted a trans conformation and formed an inclusion with R-CD in the mixture solution. As indicated by the arrows in Figure S3, there are four obvious Raman scattering peaks at 1185, 1250, 1350 and 1480 cm-1, representing the inclusion state. After UV irradiation, the azobenzene derivative adopted the cis conformation and destroyed the inclusion state, resulting in the exclusion state of the azobenzene derivative and R-CD, accordingly, and the four peaks disappeared completely. In summary, we have presented here for the first time an elegant strategy by which to attach gold nanoparticles reversibly onto MWNTs. First, an azobenzene derivative, which could form an inclusion complex with R-CD, was covalently bonded onto the surface of an MWNT, and then gold nanoparticles with R-CD capped on the surface were prepared. Because of the reversibly photocontrolled inclusion interaction between the azobenzene derivative and R-CD, gold nanoparticles capped with R-CD could be reversibly attached onto or detached from the surfaces of MWNTs. Apart from gold nanoparticles, this reversible process could be expanded to other nanoparticles, biomolecules, or therapy drugs. Thus, many applications, such as adjusting the electronic properties of MWNTs, controlling the unloading and recovery of costly metal catalysts, and controlling the release of therapy drugs, may be found. In addition, large numbers of free R-CDs on the surface of MWNTs allow further exploration in different applications. Acknowledgment. Financial support from the Doctorial Research Foundation of the Henan Agricultural University is greatly acknowledged.
3902 J. Phys. Chem. C, Vol. 113, No. 10, 2009 Supporting Information Available: FTIR spectral comparison of free thiolated R-CD and R-CD-capped gold nanoparticles, TEM image of the UV-irradiated mixture solution after centrifugation at 3000 r/min for 30 min, and Raman spectra of the mixture solution before and after UV irradiation. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) (a) Dresselhaus, M. S.; Dresselhaus, G.; Avouris, P. Carbon Nanotubes: Synthesis, Structure, Properties and Applications; SpringerVerlag Press: Heidelberg, 2001. (b) Hueso, L. E.; Pruneda, J. M.; Ferrari, V.; Burnell, G.; Valde´s-Herrera, J. P.; Simons, B. D.; Littlewood, P. B.; Artacho, E.; Fert, A.; Mathur, N. D. Nature 2007, 445, 410–413. (2) (a) Whitesides, G. M.; Mathias, J. P.; Seto, C. T. Science 1991, 254, 1312–1319. (b) Treacy, M. M. J.; Ebbesen, T. W.; Gibson, J. M. Nature 1996, 381, 678–680. (3) (a) Kong, J.; Franklin, N. R.; Zhou, C. W.; Chapline, M. G.; Peng, S.; Cho, K. J.; Dai, H. J. Science 2000, 287, 622–625. (b) Dalton, A. B.; Collins, S.; Munoz, E.; Razal, J. M.; Ebron, V. H.; Ferraris, J. P.; Coleman, J. N.; Kim, B. G.; Baughman, R. H. Nature 2003, 423, 703–703. (4) Baughman, R. H.; Zakhidov, A. A.; de Heer, W. A. Science 2002, 297, 787–792. (5) (a) Rueckes, T.; Kim, K.; Joselevich, E.; Tseng, G. Y.; Cheung, C.; Lieber, C. M. Science 2000, 289, 94–97. (b) Tans, S. J.; Verschueren, A. R. M.; Dekker, C. Nature 1998, 393, 49–52. (6) (a) Kim, Y. H.; Heben, M. J.; Zhang, S. B. Phys. ReV. Lett. 2004, 92, 176102–176102. (b) Guldi, D. M.; Rahman, A.; Sgobba, V.; Ehli, C. Chem. Soc. ReV. 2006, 35, 471–487.
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